This invention relates to electronic circuits for controlling a process monitoring system and specifically to a control circuit for a toner dispensing system in a xerographic machine. More particularly, this invention is directed to electronic circuits usable with the automatic developability control arrangement disclosed in concurrently filed application Ser. No. 773,646, in the joint names of Edward W. Vipond and Donald J. Weikel, Jr. and entitled "Developability Control Arrangement."
This invention is generally directed to improvements in developability control systems of the type described in U.S. Pat. Nos. 3,635,373 and 3,727,065, among others which employ a sensor including two parallel spaced translucent electrically conductive plates between which developer material including charged dry ink particles or toner is made to flow. The plates are coupled to an electrical circuit for producing an electric field between the plates to cause the deposition of toner on the surfaces of the plates. In these systems a high intensity developing field is maintained substantially at all times between the plates but the sense or direction of the field is reversed in direction at a preselected rate. The charged toner particles contained in the developer material flowing through the sensor are thus first attracted, then repelled from each of the plates by the reversing field. A light source is located on one side of the two plates while a first photocell is located at the other side to sense the optical density on the two plates at all operating times. The output of the first photocell is compared with the output of a second photocell separated from the first light source by a filter to cause the dispensing of toner to the developer at appropriate times based on a comparison of the output signals of the photocells.
The problem associated with prior art arrangements of the above noted type is that under certain ambient conditions a large accumulation of toner collects on the plates and remains there for an excessively long period before being removed by the cascading action of developer. More specifically, in the prior art systems, it was thought that sufficient cleaning of the plates was accomplished by periodically reversing the direction of the electric field between the plates. Thus, each cycle of the sensor included a time when one of the plates attracted toner while the other repelled it, followed by a time when the other plate attracted toner while the first attracting plate repelled it. During each sensing cycle each of the plates attracted toner particles for half the cycle time and repelled toner particles for the other half of the cycle. Cleaning, in the above prior art arrangements, was thought to be accomplished at a plate which, at the given moment, had a repelling field for toner, as determined by the polarity of the toner charge and difference in potential between the plates.
It has been found, however, that while the above noted cleaning action takes place under normal condition, under certain conditions of humidity and temperature excessive clouding of the test plates occurs despite the continued reversal of the electric field. This excessive clouding of the plates results in a low level of light transmittal therethrough to a sensing photocell which, in turn, shuts off the toner dispenser for a long period of time. Eventually, the depletion of toner in the developer mix results in a cleaning of the plates, but not until the concentration of toner is below specification resulting in poor copy quality. When the plates are eventually cleaned the system responds by turning the toner dispenser but tends to overcompensate due to severe deplation in toner resulting from the long time period it has been off. These "undershoot" and "overshoot" periods of operation lead to a temporary loss of control of the development system.
The above noted excessive clouding has been found to be more pronounced as the development plate field reversal is reduced. The frequency at which the electric fields are reversed is selected so that sufficient time is allowed for a good sample of toner to deposit on the test plates. In prior art arrangements, a frequency of approximately 1 Hz has been employed. In order to obtain a more densely developed test patch, to thereby increase the accuracy of the system, lower frequencies are required. This latter condition, while improving the accuracy of the system, has the disadvantage of more easily permitting, under certain conditions, excessive plate clouding. This latter problem is less pronounced in systems using higher rates of field reversal (about 1 Hz).